Behavioral Ecology VoL 9 No. 1: 101-108
Environmental and social influences on
calling effort in the prairie mole cricket
(Gryllotalpa major)
Peggy S. M. Hill
Department of Zoology, University of Oklahoma, Norman, OK 73019, USA
Sexual advertisement in the form of acoustic display is energetically expensive. Calling effort, or metabolic energy expended
specifically for advertisement, is adjusted in some species in response to competition from other males or to changes in climatic
conditions. The prairie mole cricket (GrjUotatpa -major) is a rare insect of die south-central United States that produces its
calling song, or advertisement rail, from a specially constructed burrow in taDgrass prairie soiL I found that prairie mole cricket
males do not adjust their call amplitude with competition or female availability, nor do they vary amplitude with soil temperature
or moisture within their established range of calling conditions. Chirp rate adjustments were made with social interactions and
changes in soil temperature, but group size did not influence calling effort Males produced more complex calls in response
to closer calling neighbors, but prairie mole cricket males may selectively respond to only the nearest one to two neighbors.
Kty words: acoustic signaling, calling effort, Gryllotalpa major, prairie mole cricket, sexual advertisement. {Bthav Ecol 9:101-
108 (1998)]
S
exual advertisement in acoustically displaying animals is
expensive in terms of the increased metabolism required
to provide energy for calling (Bailey et aL, 1993; Kavanagh,
1987; MacNally and Young, 1981; Prestwich, 1994; Wells and
Taigen, 1989). The cost of sound production is in addition to
other costs of mating (Bucher et aL, 1982; Calow, 1979), which
would include risks of predation and parasitoid attraction as
well as gamete production, e t c Acoustic advertisement by
males can account for up to 56% of the daily respiratory budget in crickets (Prestwich and Walker, 1981), but we know
little about whether anir"a'f in the field actually budget these
costs by adjusting their calling effort (Wells and Taigen, 1989)
or metabolic energy expended specifically for advertisement.
Crickets and katydids produce advertisement calls, or calling songs, by rubbing together specialized areas of the forewings. Wing-stroke rate and the number of teeth struck per
wing stroke explain most of the variation in calling costs
(Prestwich and Walker, 1981). An increase in wing-stroke rate
requires more metabolic energy (Kavanagh, 1987), as does
producing a louder call (Lee and Loher, 199S). Thus, faster
calling rates and louder calls are more energetically expensive
for individuals (Prestwich et al., 1989) and can be used as an
indirect indicator of a male's calling effort.
Social interactions may result in an increase in calling effort Male tungara frogs (Pkjsalacmus pustulosus) produce a
more complex call by adding "chuck" sounds with 12-14 harmonics of the fundamental frequency (Ryan, 1985) in response to calling by other males, but these additional vocalizations also elicit preferential phonotaxis by females (Ryan
and Rand, 1990). Male sagebrush crickets (Gyphoderris strrpiUms) appear to increase their call rate and/or intensity when
singing near others (Sakaluk et al., 1995), and the number of
competing males should affect male advertisement level under certain conditions—i.e., a sexual advertisement scramble
competition (Parker, 1984). If the number of females attracted to any given male is proportional to his level of advertiseP. S. M. Hill is now at the Faculty of Biological Sciences, University
of Tulsa, 600 South College, Tula, OK 74104, USA.
Received 25 May 1996; accepted 8 July 1997.
1045-2249/98/55.00 O 1998 International Society for Behavioral Ecology
ment relative to the mean level for all his male competitors,
and an increase in level incurs costs to his survivorship, then
a male should increase his calling effort with an increase in
competition. The evohitionarily stable strategy is an increase
in individual levels of advertisement with an increase in group
size, or male density, until costs (e.g., increased attractiveness
to predators or parasites, expenditure of energy reserves) become prohibitive (Parker, 1984). Thus, die mean advertisement level for large groups of males should be greater than
that for smaller groups because of the individual increase in
effort as male density increases.
Environmental factors may also affect these indicators of
calling effort without a shift in the individual's metabolic expenditure on calling. For example, folklore lauds a method
of measuring temperature by counting die chirp rate of crickets, and some cricket species show a linear relationship between chirp rate and temperature for an intermediate temperature range (Bennet-dark, 1989; Ciceran et aL, 1994; Doherty and Callos, 1991). Gryllotalpa vtrwai males called louder
on damper evenings, when die soils were less dry, and when
burrows were newly constructed (Bennet-Qark, 1970), and
soil moisture was die only factor besides male size that had
an influence on call intensity in ScopUriscus mole crickets
(Forrest, 1983, 1991).
I designed this study to test die following predictions concerning the relationship between calling effort and environmental or social factors: (1) individual chirp rate and/or call
amplitude will increase with an increase in competition, as
evidenced by a decrease in nearest-neighbor distance; (2) individual chirp rate and/or call amplitude will increase with
an increase in male density as group size increases; (3) mean
chirp rate and/or call amplitude per group of potential competitors will be greater at greater densities; (4) more complex
rail* will be stimulated by competition from other males and/
or availability of receptive females; (5) chirp rate will increase
with an increase in temperature; and (6) call amplitude will
increase with an increase in moisture (Table 1).
METHODS
The subject of this study, die prairie mole cricket (Gryllotalpa
major), is the largest North American cricket, measuring up
Behavioral Ecology Vol. 9 No. 1
102
T«He 1
to caffing effort <&ae to aocfaU influence* or tbet
Prediction
Reference
Result in this study
Individual chirp rate/call amplitude will increase with decrease
in nearest-neighbor distance
Individual chirp rate/call amplitude will increase with increase
in male density
Mean chirp rate/call amplitude per group of competitor! will
be greater at greater densities
More complex calls will be produced with competition and/or
female availability
Sakahik et aL (1995)
Chirp rate increased, amplitude did not
Parker (1984)
Neither increased
Parker (1984)
Neither increased
Ryan (1985); Ryan and Rand
(1990)
Chirp rate will increase with increase in temperature
Bennetdark (1970);
Ckeran et aL (1994);
Doherty and Callos (1991)
Bennet-dark (1970);
Forrest (1983, 1991)
Harmonics increased with decrease in
nearest-neighbor distance and female
availability
Chirp rate increased
Call amplitude will increase with increase in moisture
to 5 cm in length and with a mass up to 2.6 g (Walker and
Figg, 1990). It is a burrower with a historical distribution
throughout the southern tallgrass prairie of the United States
(Figg and Calvert, 1987), but all known extant populations
are in Oklahoma, Kansas, Missouri, and Arkansas. Few details
of the ecology and life history have been reported for this
rare insect, which can be located reliably only in the spring
as the male broadcasts his call at sunset.
GryUotaipa major males construct a specialized burrow from
which they produce their call (Figure 1; Walker and Figg,
1990) on spring evenings with mild temperatures and no rain
or high wind. The usually single burrow-opening in this species is highly variable in shape and tapers to a short exponential horn, much like burrow openings of other members of
the genus (Bennet-Clark, 1989). The G. major male stands
with his head in an enlarged bulb and his abdomen projecting
into the base of the horn (Hill PSM, personal observation)
like other Gryltotalpa species (Nickerson et al., 1979). Sound
is produced in mole crickets as the raised forewings (tegmina)
are rubbed together (Bennet-Clark, 1989). The airborne calling song is a series of short chirps, produced at a rate from
1.4 to 3.6/s (Hill, 1996) and composed of from 13 to 35 $yt-
Figure 1
Specialized burrow from which prairie mole cricket males produce
their advertisement call, or calling song. Males sit in the enlarged
bulb, facing away from the exponential horn. The acoustic horn
opens at the soil surface and may be obscured by grass and liner in
unburned prairies (adapted from Walker and Figg, 1990). Scale line
in diagram «= 1 cm.
Amplitude did not increase
lables (pulses) per chirp (Walker and Figg, 1990). The dominant frequency of the calling song is about 2 kHz (Walker
and Figg, 1990), with second to fifth harmonics sometimes
present (Figure 2). The acoustic burrow resembles those described by Bennet-Clark (1970) for C. vmtae, from which
sound is beamed upward to attract frying females, except that
G. vinta* has a two-horned burrow opening. The calling of
prairie mole crickets is a sexual advertisement and may largely
target flying females (Walker and Figg, 1990).
I monitored a natural population of G. major males in a
tallgrass prairie meadow at White Oak, Craig County, Oklahoma, USA (S6°37' N, 95°16' W) on all evenings from 2 April
to 21 May 1994. The meadow had been burned in mid-March
and was producing new growth by 2 April, but burrow openings were not obscured by previous years' vegetation. By listening to calls, I located burrows on all evenings of the season
when males were active. I flagged the burrow entrances and
later plotted locations on a map. CHmatological measurements were made of soil and air temperature, wind speed, sky
conditions, relative humidity and calculated saturation deficit
of the air. Soil moisture was not measured, but I assumed that
soil moisture would increase immediately after rain and decrease with number of days after a rain (Forrest, 1983).
I used minicassette recorders to document the entire calling bouts of focal males. Recorders were placed before the
onset of calling and retrieved after all calling had ceased each
evening to avoid disturbing males, which typically fall silent
when approached to within 2-3 m. I positioned recorders on
the ground at a measured distance of 20 cm from a burrow
opening, in line with the long axis of the cricket's body. Because the dominant frequency of the call is about 2 kHz
(Walker and Figg, 1990) and the position of the calling male
in the burrow is fixed (Bennet-Clark, 1970), this distance is
outside the near field for sound radiating from the burrow's
entrance but dose enough to avoid interaction of direct
sound waves from the "radiator" with waves that have been
reflected from physical features outside the burrow. Alignment is. important because the sound level in other mole
crickets at ground level is higher along the longitudinal axis
than ^jeependicujar to it (Michelsen and Nocke, 1974). The
nearest-calling neighbor was noted for each focal male recorded.
Absolute maximum call amplitude, mean chirp rate per individual per evening, and degree of call complexity, as evidenced by the number of harmonics present in a male's call,
were used as indicators of individual effort Even though the
Hill • Influences on calling effort
103
-110
-120
3
6
Frequency (kHz)
10
Figure I
Frequency spectrogram produced by Fast Fourier Transform analysis of the prairie mole cricket's airborne calling song. The dominant
frequency is at about 2 kHz, and harmonics are shown up to the fifth at just under 10 kHz.
entire calling song could be documented on tape, call duration was not a reliable measure of effort because many events
(disturbance at the burrow entrance, attraction of a female,
trains passing by, etc) caused males to cease calling. After the
disturbance, males might or might not resume calling that
evening. I analyzed recordings with SIGNAL software (Engineering Design, Belmont, Massachusetts) and generated oscillographs and sonographs from samples at about 5-min intervals after the onset of calling to measure call parameters.
I measured maximum call amplitude from the oscillograph at
the 15-min mark after onset of calling. The 15-min mark is
near the midpoint of a typical calling bout (Hill, 1996) when
males are calling rhythmically, and Bennet-Clark (1970) reported that once members of the genus are in full song, calls
vary no more than 1 dfi in intensity. I determined chirp rate
by counting chirps directly from the recording for SO s at
approximately 5-min intervals up to the 15-min mark of the
call and calculated the mean chirp rate per individual per
evening.
I estimated complexity of calling songs from the harmonics
present on the sonographs and Fast Fourier transforms that
were generated from the samples of recordings at the 5-min
mark after the onset of calling. Level 1 of complexity was assigned to calls composed almost entirely of the 2 kHz dominant frequency. Level 2 indicated calls with harmonics at 4
and 6 kHz, and level 3 included calls that had harmonics up
to 8 or 10 kHz (Figure 3).
I placed a CEL-228 Impulse Sound Level Meter and Analyzer (Computer Engineering limited, Herts, England) calibrated with a CEL-184 Acoustic Calibrator on the ground 20
cm from the burrow openings of regularly calling males, in
the same position that recorders were placed, to measure
sound pressure level (SPL) of the calling songs. The SPL meter was set to hold on the maximum RMS (root mean square)
level logged during the measurement sequence ("fast" RMS
time constanc=125 ms, dB A scale, with 0 dB re 20 uPa).
Since no measurements have been made of maximum hearing distance using G. major's auditory system, I combined observations and calculations of the range of males' calls audible
to other males to estimate the spatial limits of a potential interactive group of competitors. I used the SPL measurements
of calling songs and data from the literature on threshold
levels of hearing in other mole crickets to estimate the audible
range of G. major calls to test for a relationship between calling effort and numbers of individuals acting as potential competitors with the focal male. Based on sound pressure levels
measured at 20 cm from the males' burrow entrances (N =
30; mean - 96.1 dB; SD •* 1.4), a male's calling song is broadcast over a large area. I assumed that a practical threshold for
hearing in G. major is similar to the 40 dB level reported for
mole crickets in general (Bennet-Clark, 1989). If most attenuation of the 2 kHz component of the call in grassland, where
the vegetation is pliant and surfaces are small compared to
the wavelength of the sound, is due to geometric spreading
104
Behavioral Ecology VoL 9 No. 1
121
10-
2-
m
. • h • L.
2.0
1.0
2.5
3.0
Time (s)
121
Figure S
Sonagrams of airborne calling songs of two individual prairie mole cricket males used to assign values of call complexity based on level of
harmonics present: (a) a level 1 call with almost all energy in the 2-tHi fundamental frequency, (b) a level 3 call with higher frequency
components up through the fifth harmonic
alone (Romer and Lewald, 1992), a loss of 6 dB for each
doubling of distance away from the burrow (inverse square
law) would allow crickets more than 100 m away from the
caller to still perceive his signal (e.g., 42 dB at 102.4 m).
Although the last assumption does not take into consider-
ation factors in a complex natural environment such as wind
and temperature (Michelsen, 1978), crickets never called during the season in winds higher than 25 km/h and air temperatures lower than 193OC Variations in grass heights in early spring should not make a major contribution to impedance
Hill • Influences on calling effort
because resistance to air flow is mainly due to the turf or the
loose soil on the surface surrounding plant roots (Piercy and
Embleton, 1977). In the absence of site-specific sound transmission data, I used 100 m as a calling radius near the extreme
of a male's perception, and no burrow in the population was
located more than 100 m from its nearest neighbor (n = 76;
mean - 19.1 m; SD = 19.9; range - 1.08-91.50 m).
The establishment of a lower Emit to bracket the range of
potential competitive interaction among males was problematic. Nearest neighbors in the field were observed interacting
(by alternating calls) at a distance of 16.9 m; therefore, I
chose to use 19.1 m, the mean nearest-neighbor distance for
the population, as the calling radius of the focal male in proposing a lower limit of group size, a choice with some precedent in studying orthopteran spacing. Bailey and Thiele
(1983), studying Mygaiopsis marki, excluded individuals from
consideration (designating them as single callers) if a circle
whose radius equaled the mean nearest-neighbor distance surrounding each male did not overlap the circle of another
male.
Males producing the 73 calls used to measure maximum
call amplitude were classified into 61 groups of 1-5 individuals
(Figure 4a) based on a calling radius at the nearest-neighbor
distance (19.1 m). On a map of the population, a circle of
radius 19 m was drawn around each focal male's burrow position, and only those males within the circle and calling on
the night the recording was made were considered to be in
that male's group of potential competitors. On succeeding
evenings, a focal male could be in different-cized groups because not all males called on every night of chorusing, and
burrows were opened and abandoned throughout the season.
A group mean maximum call amplitude was calculated for
each group of males (from the evening's recordings of individuals in the group) so that call amplitudes of groups of different numbers of males could be compared. Many, but not
all, of the group means were based on recordings of all group
members calling that evening. When the calling radius was set
at 100 m, these males were classified into 42 groups of 2 to
26 individuals (Figure 4b). Means of groups of 16 or more
members were calculated from recordings of 5-6 group members.
I assigned males that had produced the 77 calls used in
analyzing mean chirp rate to potential interactive groups in
the same way I did when examining the relationship between
call amplitude and group size. Using an estimated call radius
of 19.1 m, I classified males into 64 groups of 1-5, and at 100
m in 44 groups of 2-26 individuals.
I calculated the mean amplitude and mean chirp rate for
each potential interactive group based on these upper and
lower limits of estimated group size and compared each of
these means with the means of the other potential groups. No
assumptions were made before the gathering of data that a
G. major male would adjust his calling effort in response to
competitors simply because he could perceive them. In fact,
evidence from other orthopteran species has established a pattern that males space themselves at less than half the maximum hearing distance (Romer and Bailey, 1986) and typically
ignore all but the one or two closest or loudest neighbors
(Greenfield, 1994).
The burrow openings of prairie mole cricket males are
sealed the morning after a female has been attracted, as evidenced by the male's courtship song, even though adults are
no longer found in the acoustic burrow by the next morning
(Hill, 1996). I used sealed burrows as evidence of availability
of receptive females during the course of the calling season
(Figure 5) , assuming that matings increased as numbers of
receptive females increased on site, even though a census of
females was not made.
105
a
1
2
3
4
5
Group Size (Caffing Radius 19.1 m)
3
6
9
12
15
18
21
24
Group Size (Cafflng Radius 100 m)
Figure 4
Number of individuals calling per group one when the calling
radius was estimated at (a) nearest-neighbor distance of 19.1 m (n
m
61); and (b) 100 m estimated maximum transmission distance
for prairie mole cricket advertisement calls (n «= 42).
I used SigmaStat statistical software (Jandel Scientific San
Rafael, California) to analyze all data. The level of association
between paired variables was tested with Spearman rank order
correlation after all sets of data for individual variation failed
the Kolmogorov-Smimov test of normality. The Kruskal-Wallis
one-way ANOVA on ranks was used to test for variation in
mean call amplitude of groups of individuals as density increased when data were not normally distributed.
RESULTS
In 1994, males of the White Oak Prairie population of G.
major called on only 22 of the 37 evenings from 13 April
through 19 May, and I recorded acoustic activity at burrow
106
Behavioral Ecology Vol. 9 No. 1
240
100 -
80 -
60 -
20 -
0 6
8
10
12
8
7
Figure 5
Percentage of male burrow openings closed after courtship by date
(n - 15) of calling season. This relationship reflect* female
availability by date.
entrances on 15 of these nights. I recorded 77 calling songs,
representing 27 different burrows, and used these to calculate
chirp rates. Only 73 of the calls, from 26, burrows, were of at
least a 15-min duration, and these were used to determine
maximum call amplitude. I made IS recordings at a single
burrow over the 15 nights of sampling calls, but 73% of the
burrows were sampled on only 1-3 nights.
Absolute maximum amplitude levels of the 73 calling songs
varied from 0.323 to 139 volts (mean => 1.159, SD =• 0.27),
or approximately 90-104 dB, at the 15-min mark. Individual
call amplitude was not related to the distance between a male
and his nearest calling neighbor (r ™ —.142, p = .231), soil
temperature (r =• —.684, p — .477), number of days since it
had rained (r >» -.132, p - .263), or availability of receptive
females (r - .080, p « 302). Individual call amplitude was
not linked to density of potential competitors when the interactive group was limited to a circle with radius of either 19.1
m (r = .026; p - .825) or 100 m (r - -.145; p = .220).
Mean chirp rate per individual per night varied from 78.0
to 216.0 (943 to 226.1 when corrected to 10°O chirps per
minute (n = 77, mean •» 166.67, SD «• 30.63) over the 15
nights of sampling. Chirp rate for the focal male tended to
increase with a decrease in the nearest-neighbor distance, although the probability level did not meet the standard level
of statistical significance (r » —.208, p • .070). Chirp rate
increased significantly with soil temperature (Figure 6), soil
moisture (r «• .355, p = .002), and the availability of receptive
females, as reflected by increased burrow closing (r » .271, p
- .018). Mean individual chirp rate was not related to group
size when the mean nearest-neighbor distance was used to
define the potential interactive group (r «• .031; p • .789),
but when a radius of 100 m around the focal male was mod
to define the group, mean individual chirp rate increased significantly with an increase in density of potential competitors
(r » .305; p => .007). However, when outliers from the 19 and
20 April samples that were identified through a residuals analysis were omitted, the relationship between mean individual
chirp rate and group size at 100 m was not significant (n =
8
9
10
11
12
13
14
Sol Temperature (C)
Day of CaBng Season
Figure 6
Mean chirp rate per Individual per night versus soil temperature (n
- 77, r - .71,p<Ml).
65; r ~ .191, p ~ .127). Maximum call amplitude and mean
chirp rate per individual per night were not related (n = 73;
r -» .057, p - .629).
Mean call amplitude did not vary with male density (Kruskal-Wallis one-way AN OVA on ranks: 19.1-m limit, p » 399
and 100-m limit, p — .165). There was no significant variation
in mean chirp rate based on group size at either 19.1 m (oneway ANOVA: p - 339; but power = 0.050) or at 100 m (Kruskal-Wallis one-way ANOVA on ranks: p - .281).
The level of harmonics in the calling song produced by
individual males increased with a decrease in distance to the
nearest-calling neighbor (r ™ —.264, p = .020) and with maximum call amplitude (r •= .380, p<.00\). The relationship
between harmonics and the availability of females fell just
short ef statistical significance (r = .218, p • .057). There was
no correlation between level of harmonics and mean individual chirp rate (r «• .038, p *• .743), soil temperature (r =
.088, p ™ .446), or number of days since the last rain (r »
.005,/> = .966).
DISCUSSION
Field workers monitoring prairie mole cricket populations
routinely comment that individual males are louder when
they are near others or on nights when more males are calling. This observation makes intuitive sense—males might be
assumed to respond to competition for mates by increasing
their effort in advertisement up to some level where costs exceed benefits. However, prairie mole cricket males did not
increase maximum call amplitude individually with an increase in competition from other males, nor was call amplitude altered in response to (or in conjunction with) availability of receptive females. There was no significant relationship between a focal male's caH amplitude aaddistance to the
nearest calling neighbor. More dense aggregations of males
did not call louder than smaller groups, and individuals did
not increase amplitude of their calls with density when the
calling radius was set at the mean nearest-neighbor distance
for the population. Human perception of louder calling when
Hill * Influences on calling effort
males are closer together may simply be due to a perceived
additive, or psychoacoustic, effect. A similar perception of increased effort in the sagebrush cricket (Cyphoderris stnpitans)
was not supported by experiments testing the importance of
male-male interactions to spacing or mating success (Sakahik
et aL, 1995).
Indeed, studies showing female preference for, and higher
mating success for, louder males (e.g., Forrest, 1983; Forrest
and Green, 1991; Gerhardt, 1987; Walker 1983) were not designed to address the issue of whether males were actually
increasing their effort with competition. Males may typically
call at a maximal individual level, but some are able to sustain
a higher intensity call because of higher fitness. Selection
would then favor the ability to produce loud calls rather than
a flexibility in behavior that allows for modification of effort
with competition.
Call amplitude of individuals was not linked to soil temperature or soil moisture. Forrest (1983, 1991) found that toil
moisture significantly influenced call intensity in Scapttriscus,
but Ulagaraj (1976) found no association with air. or soil temperature in the same genus. The Parsons soil on the White
Oak site is poorly drained and has a clayey subsoil (USDA,
1973), and it was consistently moist throughout the season, so
that soil moisture was not likely a factor lirniring call intensity
at White Oak Prairie.
Calling effort measured in terms of mean chirp rate per
individual was significantly linked to the social environment.
Individuals tended to call faster when nearest calling neighbors were closer and when more females were available. However, males did not predictably call faster in more dense aggregations, whether die calling radius was set at die mean
nearest-neighbor distance or at die estimated maximum transmission distance of the calling song. Mean individual chirp
rate did not increase with an increase in male density. Other
orthopteran males that alternate their calls with those of nearby conspecifics modify chirp rate during interactions: Pteropkylla cameUifoUa sings 20% slower, but PhoHdoptem gristoapttra sings faster (Greenfield and Shaw, 1983). The gray treefrog (Hyla venicoior) increased call duration and decreased
call rate in response to competition (Wells and Taigen, 1986).
Tarbrush grasshoppers (LiguroUttix planum) selectively respond to only their one or two closest calling neighbors
(Minckley et al., 1995). If prairie mole crickets behave similarly, they could be stimulated by a decrease in nearest-neighbor distance without being stimulated to call faster, incrementally, if potential competitors increased beyond two or three.
In odier words, if males are influenced by only dieir nearest
calling neighbor, increasing density in die neighborhood without decreasing the distance to the nearest male may have no
effect
Individual males called faster with an increase in soil temperature and moisture. The link between chirp rate and soil
moisture is not an obvious one, but more Scapteriscus males
called with an increase in soil moisture (Kleyla and Dodson,
1978). Because soil moisture at White Oak Prairie was likely
not limiting during the calling season, the correlation between chirp rate and moisture may simply reflect an interconnection with social influences.
Based on soil temperature alone, G. major individuals do
not behave differendy dian odier orthopteran species where
males use increased call rate to increase access to females (Ciceran et aL, 1994; Doherty and Callos, 1991; Ulagaraj, 1976).
Soil temperature and chirp rate were strongly related at a
highly significant level, and this is a common relationship
seen also in anurans between call rate and air temperature
(see Gerhardt, 1991). It may take die same amount of metabolic energy to call fast at a high temperature as to call slowly
at a lower temperature, and that part of the increase in call
107
rate explained by temperature alone would not require an
increase in calling effort However, metabolic costs do sometimes increase widi ambient temperature, and die relationship is a complicated one (Prestwich, 1994).
More complex calls, measured by numbers of harmonics in
die calling song, were produced when nearest calling neighbors were closer and females were more available, but call
complexity was not related to environmental ipfhiwcnt like
soil temperature or moisture. More complex calls tended to
be louder but not faster, suggesting that diere are trade-offs
at some leveL Louder calls are perceived to be from closer
males, even when diey are not (Forrest and Green, 1991).
Likewise, because harmonics (which have less energy dian die
dominant frequency) attenuate over short distances, a call loses its higher frequencies widi distance. A male producing a
more complex call widi more harmonics might be perceived
as closer in space to die female dian would a nearer male
widi fewer harmonics in his calL Female bushcrickets, R*qutna verticalis, passively choose die closest male based on die
loudness of his call or die power at die higher frequencies of
his call (Bailey et aL, 1990). Female katydids, Scuddtria cunticauda, prefer males diat sing a more complex song of longer
phrases or more syllables per phrase (Tuckerman et al., 1993).
Number of harmonics as one component of a call has been
of interest in studying bodi anurans and insects, but generally,
number of harmonics alone has not been examined in terms
of male-male competition or female choice (see Ryan et aL,
1990). Further study is needed to determine if die level of
harmonics in G. major's advertisement call plays an important
role in male-male interactions and in increasing attractiveness
to females.
In conclusion, prairie mole cricket calling effort can be predicted based on some comparisons widi acoustically displaying
males from die literature, but die observed behavior is not
consistent widi all comparisons. Like die tungara frog (Ryan
and Rand, 1990), males produce more complex calls widi
competition, even diough die sources of die complexity of
die call components studied in die two species are quite different. Chirp rate increases widi temperature and widi competition, but not widi group size. Calling effort cannot be predicted by theory from Parker's (1984) sexual advertisement
scramble model, but it is not clear diat die assumptions on
which Parker's model are based were actually met by this mating system. Even diough acoustically displaying prairie mole
cricket males are aggregated spatially (Hill, 1996), individual
males may be selectively responding to only those nearest one
or two calling neighbors, and an increase in group size beyond these would not be important to any aspect of calling
effort measured. Call amplitude was not influenced predictably by any social or environmental factors measured: not soil
moisture, nor competition through decreased intermale distance, nor increased male density. Prairie mole cricket males
appear to call at their own inherent individual capacity on
evenings when diey do rail, and future work may clarify
whether diis is due to body size or some other physical characteristic
I thank Marc Mangel and two anonymous reviewers for thoughtful
comments and direction in preparing this manuscript for publication
and Victor Hutchison, James Thompson Jr. .James Estes, Gary Schnell
and Patricia Schwagmeyer for presubmisiion criticism. I thank Kenneth Prejtwich, Carl Gerhardt, Michael Greenfield, Thomas Walker,
DennU Figg, and Timothy Forrest for answering many questions and
Bobi Jo Hill for valuable field assistance. I thank Wallace Olsen and
the Kelly Ranch for freedom to work at White Oak Prairie. The study
was supported by the U.S. National Biological Service (contract 1444-0009-94-1001). a VS. Department of Education CAANN Fellowship, The Nature Conservancy's Tulsa Field Office, and a Katherine
Ordway Stewardship Grant (The Nature Conservancy).
108
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